Some induction motors, used as, e.g., spindle motors for machine tools, have squirrel-cage rotors which include secondary conductors and end rings both integrally formed with a laminated core by a casting process. When the speed of an induction motor having such a squirrel-cage rotor must be increased, there is the concern that the end rings arranged at the axial ends of a laminated core may bend or break due to the centrifugal force caused by high-speed rotation. A known squirrel-cage rotor adaptable to high-speed rotation includes separate reinforcing members formed of high-rigidity materials, such as iron or stainless steel, the reinforcing members being fixedly disposed on the rotor while surrounding the end rings, so as to eliminate the deformation of the end rings.
Conventional reinforcing members for the end rings of a high-speed squirrel-cage rotor are formed as annular elements, each of which has a generally L-shaped cross-section at a circumferential region and includes a cylindrical wall coming into contact with the cylindrical radial outer circumferential surface of the end ring and an end wall coming into contact with the axial end surface of the end ring. The reinforcing member covers the outer surface of the end ring, and the end wall thereof is usually fixed to the axial end surface of the end ring by fixing means such as bolts. Therefore, deformation of the end ring due to centrifugal force is prevented by the cylindrical wall of the reinforcing member. In such a construction, however, the cylindrical wall is supported through the end wall on the end ring in a cantilever manner, which causes the problem that the cylindrical wall tends in itself to be bent outward due to centrifugal force in response to the increase of the diameter of the rotor or the rotation speed thereof, and thus fails to support the end ring.
The applicant of the present invention provided in the International Patent Application No. PCT/JP93/01335 (co-pending application), as a related art for solving the above problems, a squirrel-cage rotor comprising reinforcing members, each of which includes a cylindrical wall for surrounding a cylindrical outer surface of each end ring, a multi-aperture wall provided with a plurality of apertures which communicate respectively with the through holes of the laminated core for secondary conductors and extending in a radial inward direction from one edge of the cylindrical wall so as to be held between the end ring and an axial end face of the laminated core, and an end wall extending in a radial inward direction from the other edge of the cylindrical wall so as to come into contact with an axial outer surface of the end ring at a side opposite to the multi-aperture wall. In the casting process of this squirrel-cage rotor, a pair of reinforcing members is first disposed at and brought into contact with the axial end faces of the laminated core so that the apertures of the multi-aperture walls of the reinforcing members respectively communicate with the through holes of the laminated core. In this state, the laminated core and the reinforcing members are supported by a jig. Then, the laminated core and reinforcing members supported by the jig are placed in a mold, and molten metal is poured into a center opening of the end wall of one reinforcing member, so as to cast a plurality of secondary conductors and a pair of end rings. In this manner, the reinforcing members are fixedly held by the end rings and the laminated core, and the cylindrical walls of the reinforcing members are firmly supported at both ends thereof through the multi-aperture walls and the end walls, and thus the reinforcing members themselves are prevented from being deformed.
In the casting process of the reinforcing members for end rings according to the above related art, it is required to smoothly discharge air from a casting cavity in order to stabilize the flow of molten metal and to improve the quality of the casting. To this end, it may be proposed that a plurality of air vent holes can be provided in the other reinforcing member which is located opposite to a gate. However, from the viewpoint of ensuring the strength of the reinforcing member, it is preferred that the air vent holes are not provided in the reinforcing member.